Patent document JP H09-229901A discloses an oxygen concentration detection device, which includes a limiting-current type oxygen sensor for detecting an air-fuel ratio of exhaust gas, a bias control circuit for supplying a voltage to the limiting-current type oxygen sensor, and a current detection circuit for detecting a current flowing in the limiting-current type oxygen sensor. The limiting-current type oxygen sensor and the bias control circuit are connected electrically through two conductive wires, which are connected to an exhaust gas-side electrode and an atmospheric air-side electrode of the limiting-current type oxygen sensor, respectively. A voltage is supplied from the bias control circuit to the limiting-current type oxygen sensor through the two conductive wires. With this voltage application, a current flows in the limiting-current type oxygen sensor. The current detection circuit includes a current detection resistor, which is provided in the conductive wire. An air-fuel ratio is detected based on the current flowing in the current detection resistor.
In the oxygen concentration detection device disclosed in the above-referenced patent document, grounding wires are connected to the conductive wires, which electrically connect the limiting-current type oxygen sensor and the bias control circuit, respectively. A capacitor is provided in the grounding wire. The current detection resistor is provided in the conductive wire. According to this configuration, since a part of the current flowing in the limiting-current type oxygen sensor flows into the capacitor, the current (sensor current) flowing in the limiting-current type oxygen sensor cannot be detected with high accuracy.